One of the great mysteries of the universe is what is dark matter. This bizarre material doesn’t interact with light but gravitationally throws light and mass around as it interacts with things at galaxy and galaxy cluster scales. People keep trying to identify dark matter with specific theorized particles and keep looking for new ways to make it go away with new observations. So far, dark matter refuses to be seen and refuses to go away.
In the 1990s, there was some evidence that dark matter might be Weakly Interacting Massive Particles or WIMPs. This came from an Italian experiment that looked directly for WIMPs colliding with sodium iodide in a detector. It was expected that if WIMPs are dark matter, then they will be everywhere, and the detector would act as a windshield in rain, catching the WIMPs as we go through the galaxy. Specifically, it was expected that the Earth’s motion would seasonally change the rate at which WIMPs are detected as we move with and against the Sun’s motion around the Sun, just as the amount of rain hitting a windshield varies with velocity relative to the storm.
In 1998, the Italian team announced the detection of the expected signal, and in the twenty-plus years since, teams around the world have tried to find further evidence of WIMPs, to no avail. Most recently, a Spanish team – ANAIS collaboration – replicated the experiment in its entirety and found nothing, proving once again that to truly believe a result, you need to replicate that result. This seems to indicate WIMPs are not the dark matter particle we need.
This lack of detections brings into question more than just the possibility of dark matter being WIMPs. WIMPs are a key prediction of the supersymmetry model of particle physics. No WIMPs could mean no supersymmetry, which means that a whole lot of other predicted particles, for which we admittedly have no evidence, also don’t exist.
I personally am not bothered by this. The universe is what it is, and we’ll figure it out when we figure it out. There are, however, a lot of physicists who want there to be an underlying theory that explains why our universe has the specific set of particles that it has. Supersymmetry is one of the more advanced accepted possible theories that try to explain why we have electrons, protons, and neutrons; why there are the quarks we see and not some other set. It is a complex theory I can’t pretend to say I understand, and now it is looking more and more like it is a theory that doesn’t work.
And that puts us back to having a universe that exists for reasons we just don’t know.
“Annual Modulation Results from Three Years Exposure of ANAIS-112,” J. Amare et al., 2021 March 1, arxiv.org